On the montane species of Kodaikanal, South India

Meher-Homji, V. M.

Abstract

The zonation of vegetation is discussed in relation to floristic composition, altitude and climatic factors. Two explanations are advanced for the distribution of the species of the tropical deciduous forest located at 800-1300 m. (1) A cooler climatic phase in the past enabled these species to thrive at lower elevation presently occupied by the Albizia amara thorn forest community. (2) The climate remained unchanged but the diaspores reached the 800-1300 m belt from the eastern fringe of the W. Ghats. For the montane species, mostly endozooically dispersed, occurring above 1500 m, arguments are advanced for a slow rate of migration during the Pleistocene glaciations rather than long-distance dispersal by birds and winds from the Himalayas. Present-day distribution of montane forest genera like Pittosporum, Symplocos, Elaeocarpus, etc. which also occur at lower altitude in W. and E. Ghats suggests step by step extension through speciation and provides a clue to the actual disjunct distribution of the montane shrub-savanna genera Rhododendron, Mahonia, Gaultheria distributed in the Himalayas on the one hand and hills of S. India and Sri Lanka on the other. Warming up of the climate would eliminate the montane forest genera from the lower elevation and the resultant distribution would resemble the present-day pattern of Rhododendron, Mahonia.